An organic light emitting diode (OLED) is a self-luminous device that emits light when current is applied. OLEDs have a response speed more than one thousand times faster than that of LCDs, and have a wide viewing angle. OLEDs do not require the backlight, color filter, etc., required by LCDs and can be implemented with a flexible display such as plastic, and thus have great advantages not only in performance but also in the cost of parts.
The emission principle of OLEDs may be briefly described as follows. The holes injected at the anode and the electrons injected at the cathode are reconnected in the light emitting layer to form excitons, and as the excitons return to a stable state, the emitted energy is converted into light, and thereby light is emitted. OLEDs are classified into fluorescence and phosphorescence types according to the principle of luminescence. As described in the emission principle of OLEDs, holes and electrons are reconnected to form excitons, and in particular, the excitons in a “singlet excited state” and those in a “triplet excited state” are present in a quantum mechanical ratio of 1:3. In fluorescent materials, energy is converted to light only in the “singlet excited state”, while energy is consumed as heat in the “triplet excited state”, and thus the maximum internal quantum efficiency is limited to 25% or less. However, in phosphorescent materials, as the “singlet excited state” becomes stable by undergoing the “triplet excited state”, the energies in both states can be converted to light, thereby achieving an internal quantum efficiency of up to 100%. Such phosphorescence is possible due to molecular structural features. Typically, the energy conversion from the “singlet excited state” to the “triplet excited state” is made possible through a coordination structure in which a metal ion such as iron, ruthenium, iridium, platinum, etc., is inserted in a molecular structure and organic materials are connected in the neighboring area.
However, metals such as iron, ruthenium, iridium, platinum etc., are very expensive and relatively difficult to use.
Boron atoms are less toxic than other heavy metal atoms such as iron, ruthenium, iridium, platinum etc., are relatively easy to use, and have many advantages in biological use. Boron-containing compounds exhibit unique optical properties. They can efficiently emit light via strong quantum absorption at low temperatures and are being studied and developed for use in lasers, molecular probes, and phosphorescent materials.
In particular, since a four-coordinated organic boron compound having a rigid π-conjugated structure has strong luminescence and high carrier mobility, it can be applied to organic light emitting diodes (OLEDs), organic field transistors, photoreactive sensors, and optoelectronics including imaging materials. Various chelating ligands and boron moieties have been developed to construct electronic structures and molecular arrangements that are suitable for playing an important role in the photophysical and electrical properties of four-coordinated boron compounds. As a result of these efforts, many molecules exhibiting high performance as light-emitting materials have been developed (Four-coordinate organoboron compounds for organic light-emitting diodes, Chem. Soc. Rev. 2013, 42, 8416). Meanwhile, multidisciplinary research on novel organic luminescent dyes is underway for potential applications in plastic electronics and biomedicine. In particular, attempts have been made to surround the center of tetrahedral boron(III) with fluorescent dyes (Luminescent Materials: Locking p-Conjugated and Heterocyclic Ligands with Boron (III), Angew. Chem. Int. Ed. 2014, 53, 2290).
Conventionally, four-coordinated organic boron compounds were generally synthesized by adding a diarylborinic acid (Ar2B—OH), a diarylborinic anhydride (Ar2B—O—BAr2), or a triarylborane (Ar3B) to a two-coordinated N,O-ligand (Qingguo Wu et al., Chem. Mater. 2000, 12, 79-83; WO Publication WO 02/44184 A2; Yi Cui et al., Inorg. Chem., 2005, 44, 601 to 609; Norberto Farfan et al., J. Chem. Soc., Perkin Trans 2, 1992, 527 to 532; Zuolun Zhang et al., Inorg. Chem. 2009, 48, 7230 to 7236).
However, in the case of the above three compounds, various derivatives are not commercially available, and thus these compounds are obtained by synthesis in the laboratory. Therefore, the synthesis of boron compounds with the required optical properties is very limited.
Specifically, 8-quinolinolato-bis(2-benzothienyl)borane is synthesized from 8-hydroxyquinoline by adding tris-(2-benzothienyl)borane. In particular, since tris-(2-benzothienyl) borane is not commercially available, it must be obtained experimentally from 2-benzothiophene by a two-step reaction. Additionally, both n-BuLi and BBr3 are expensive and very dangerous reagents to handle.
